The invention relates to a process for preparing alkyl mercaptans by catalytic gas phase reaction of alkanols and hydrogen sulphide over alkali metal tungstates, the reaction being performed in at least two successive reaction zones which contain catalysts of different activity and selectivity.
Among the alkyl mercaptans, methyl mercaptan in particular is an industrially important intermediate, for example for the synthesis of methionine and for the synthesis of dimethyl sulphoxide and dimethyl sulphone. It is nowadays prepared predominantly from methanol and hydrogen sulphide by reaction over a catalyst composed of aluminium oxide. Methyl mercaptan is synthesized usually in the gas phase at temperatures between 300 and 500° C. and at pressures between 1 and 25 bar.
As well as the methyl mercaptan formed, the reaction mixture comprises the unconverted starting materials and by-products, for example dimethyl sulphide and dimethyl ether, and also gases, for example methane, carbon monoxide, hydrogen and nitrogen. The methyl mercaptan formed is removed from this reaction mixture.
For the economic viability of the process, a very high yield is required in the catalytic reaction of methanol and hydrogen sulphide to give methyl mercaptan, in order to keep the complexity in the removal of the methyl mercaptan formed from the reaction mixture as low as possible. The energy expenditure in particular for the cooling of the reaction mixture to condense the methyl mercaptan constitutes a large cost factor here.
To increase the activity and selectivity, aluminium oxide as a support is typically admixed with potassium tungstate or caesium tungstate. The tungstate is usually used in amounts up to 25% by weight, based on the total weight of the catalyst. An improvement of activity and selectivity is also obtained by increasing the molar ratio of hydrogen sulphide to methanol. Typically, molar ratios between 1 and 10 are employed.
However, a high molar ratio also means a high excess of the hydrogen sulphide in the reaction mixture and hence the necessity of conducing large amounts of gas in circulation. To reduce the energy expenditure required for this purpose, the ratio of hydrogen sulphide to methanol should therefore differ only slightly from 1.
EP 0 832 687 B1 describes the advantages of using caesium tungstate (Cs2WO4) instead of potassium tungstate (K2WO4) as a promoter. For instance, use of caesium tungstate allows an enhanced activity with simultaneously good selectivity to be achieved.
Increasing the caesium tungstate concentration to up to 40% by weight allows the selectivity for methyl mercaptan to be enhanced to up to 92% without the activity deteriorating disproportionately.
According to the general view, the best selectivity is achieved with catalysts in which the alkali metal/tungsten ratio is equal to 2:1 (A. V. Mashkina et al., React. Kinet. Catal. Lett., Vol. 36, No. 1, 159-164 (1988)).
These investigations do not take into account that the concentration ratios of reactants and products, but also temperatures in the reactor, differ greatly in the course of the reaction.
It is an object of the present invention to provide a process which features an improved yield over the known processes and hence leads to higher economic viability.